IP Address Assignment

An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network. A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection.

IP Address Details

Divided into two parts– Prefix identifies network

– Suffix identifies host

Global authority assigns unique prefix to network

Local administrator assigns unique suffix to host

Original Classes Of Addresses

Initial bits determine class

Class determines boundary between prefix and suffix

Dotted Decimal Notation

Shorthand for IP address

Allows humans to avoid binary

Represents each octet in decimal separated by dots

NOT the same as names like www.somewhere.com

Example Of Dotted Decimal Notation

Four decimal values per 32-bit address

Each decimal number– Represents eight bits

– Is between 0 and 255

Classful Addresses And Network Sizes

Maximum network size determined by class of address

Class A large

Class B medium

Class C small

Addressing Example

Illustration Of Router Addresses

Address prefix identifies network

Need one router address per connection

Special Addresses

Network address not used in packets

Loopback never leaves local computer

Subnet And Classless Addressing

Not part of original scheme

Invented to prevent address exhaustion

Allow boundary between prefix and suffix to occur on arbitrary bit boundary

Require auxiliary information to identify boundary

Address Mask

Accompanies IP address

32 bit binary value

Specifies prefix / suffix boundary– 1 bits cover prefix

– 0 bits cover suffix

Example: class B mask is

255 . 255 . 0 . 0

Subnet Addressing

Goal: extend address space

Invented in 1980s

Works within a site

Technique– Assign single network prefix to site

– Divide suffix into two parts: network at site and host

Typical use: divide class B address

Example Of Subnet Addressing

Single Class B number such as 128.10.0.0 assigned to site

Site chooses subnet boundary such as 24 bits

Routers and hosts configured with corresponding subnet mask

M = 255.255.255.0

Given destination address, D, extract prefix with ‘‘logical and’’ operation

D & M

Classless Addressing

Goal: extend address space

Invented in 1990s

Works throughout Internet

Accommodates– Original classful addresses

– Subnet addresses

– Other forms

Classless Addressing (continued)

Technique– Allow arbitrary prefix size

– Represent network address as pair ( address , mask_size )

Known as Classless Inter-Domain Routing (CIDR)

CIDR

Uses slash notation

Example

128.211.0.0/17

means that the boundary between prefix and suffix occurs after the first 17 bits.

Each network can be as large or small as needed (power of two)

Motivation ForIP Packets

Because it can connect heterogeneous networks, a router cannot transmit a copy of a frame that arrives on one network across another. To accommodate heterogeneity, an internet must define a hardware-independent packet format.

Internet Packets

Abstraction

Created and understood only by software

Contains sender and destination addresses

Size depends on data being carried

Called IP datagram

The Two Parts Of An IP Datagram

Header– Contains destination address

– Fixed-size fields

Payload– Variable size up to 64K

– No minimum size

Datagram Header

Three key fields– Source IP address

– Destination IP address

– Type (contents)

IP Datagram Forwarding

Performed by routers

Similar to WAN forwarding– Table-driven

– Entry specifies next hop

Unlike WAN forwarding– Uses IP addresses

– Next-hop is router or destination

Example Of An IP Routing Table

Table (b) is for center router in part (a)

Routing Table Size

Because each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in an internet.

Datagram Forwarding

Given a datagram

Extract destination address field, D

Look up D in routing table

Find next-hop address, N

Send datagram to N

Key Concept

The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header.

IP Semantics

IP is connectionless– Datagram contains identity of destination

– Each datagram sent/handled independently

Routes can change at any time

IP Semantics(continued)

IP allows datagrams to be– Delayed

– Duplicated

– Delivered out of order

– Lost

Called best effort delivery

Motivation: accommodate all possible networks

PART XII

Internetworking Part 4

(Transport Protocols, UDP and TCP, Protocol Port

Numbers)

Transport Protocol

Separate layer of protocol stack

Conceptually between– Applications

– IP

Terminology

IP– Provides computer-to-computer communication

– Source and destination addresses are computers

– Called machine-to-machine

Transport protocols– Provide application-to-application communication

– Need extended addressing mechanism to identify applications

– Called end-to-end

Transport Protocol Functionality

Identify sending and receiving applications

Optionally provide– Reliability

– Flow control

– Congestion control

Note: not all transport protocols provide above facilities

Relationship Between TransportProtocols And Other Protocols

Transport protocols are end-to-end

Transport protocol on one computer uses IP to communicate with transport protocol on another computer

Two Transport Protocols Available

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)

Major differences– Interface provided to applications

– Underlying functionality

User Datagram Protocol

Lightweight transport

Becoming more popular (IP telephony)

Best-effort delivery

UDP Features

Connectionless service

Arbitrary interaction

Message-oriented interface

Best-effort semantics

Each message encapsulated in IP datagram

Uses protocol ports to identify applications

UDP Details

Accepts and delivers messages– Message received is exactly same as message

sent

– Boundaries preserved

Maximum message size approximately 64K octets Efficient– No connection overhead

– No state information maintained

UDP Semantics

Same best-effort semantics as IP (i.e., unreliable transfer)

Message can be– Lost

– Duplicated

– Delayed

– Delivered out of order

Works best in LAN applications

Interaction With UDP

UDP allows communication that is– 1-to-1

– 1-to-many

– Many-to-1

– Many-to-many

Application programmer chooses

Packet Delivery

UDP can support– Unicast

– Multicast

– Broadcast

User Datagram Format

Extremely small header (called thin layer)

Checksum optional

UDP Encapsulation

Two levels of encapsulation

UDP datagram size cannot exceed maximum IP payload

Identifying An Application

Cannot extend IP address– No unused bits

Cannot use OS-dependent quantity– Process ID

– Task number

– Job name

Must work on all computer systems

Identifying An Application(continued)

Invent new abstraction– Called protocol port number

– Used to identify sending or receiving application unambiguously

– Independent of underlying operating system

– Used only with TCP/IP protocols

Protocol Port Numbers

Server– Follows standard

– Always uses same port number

– Uses low port numbers

Client– Obtains unused port from protocol software

– Uses high port numbers

Protocol Port Example

Domain name server application is assigned port 53

Application using DNS obtains port 28900

UDP datagram sent from application to DNS server has– Source port number 28900

– Destination port number 53

When DNS server replies, UDP datagram has– Source port number 53

– Destination port number 28900

Transmission Control Protocol (TCP)

Major transport protocol used in Internet

Heavily used

Completely reliable transfer

TCP Features

Connection-oriented service

Point-to-point

Full-duplex communication

Stream interface

Stream divided into segments for transmission

Each segment encapsulated in IP datagram

Uses protocol ports to identify applications

TCP Feature Summary

TCP provides a completely reliable (no data duplication or loss), connection-oriented, full-duplex stream transport service that allows two application programs to form a connection, send data in either direction, and then terminate the connection.

Apparent Contradiction

IP offers best-effort (unreliable) delivery

TCP uses IP

TCP provides completely reliable transfer

How is this possible?

Achieving Reliability

Reliable connection startup

Reliable data transmission

Graceful connection shutdown